Welcome to the forum, Fritz.
As stated in the note at the top of the manual page on deleting rests, you cannot (currently) remove/delete rests from unpitched percussion. Deleting rests
For clarity: starts/ends voice is exactly the same functionality, so that won’t work either.
This limitation is theoretically being addressed in the next major version cycle of Dorico (e.g. not confirmed as Dorico 4.0 but if not 4.0 then 4.1 or 4.2 or whatever).
In the meantime, the only good solution is to use the smallest number of real voices you can, and then switch notes into extra voices (which don’t create rests in the first place) from Edit (or right-click) > Percussion > Change Voice > Extra Voice.